1. Field of the Invention
The present invention is directed to an x-ray image intensifier of the type having an evacuated housing, an input luminescent screen, electron optics, and an image sensor disposed inside the housing at the side of the housing disposed opposite the input luminescent screen.
2. Description of the Prior Art
An x-ray image intensifier of the type described above is disclosed in European Application 0 083 240, which is used as part of a medical x-ray examination apparatus. The x-ray image intensifier supplies two-dimensional transillumination exposures in the form of video images. The x-ray quanta are absorbed in a scintillator of the input luminescent screen, and are thereby converted into light. The emitted light quanta release electrons in a photocathode of the input luminescent screen. These electrons are accelerated in the electrical field of the electron optics, and are focused onto an image sensor which converts the electron image into a video image, and supplies corresponding video signals. The video signals can then be further processed in a digital imaging system or can be used for video image presentation. Image sensors such as solid-state image pick-ups are employed in such known devices which are usually based on the charge-shift principle (CCD), and are suitable in their standard embodiment for the documentation of photons in the visible range. Backside-thinned CCDs can be employed for the detection of electrons.
By contrast to photons, electrons leave effects along their entire passage through a material. In the electron irradiation of a CCD from the front side, the extremely thin, insulating layer is also affected. This layer may consist, for example, of SiO.sub.2 and separates the conductive shift structures (gates) from the semiconductor substrate. The demands made of this insulating layer are extremely high because of the high field strengths which are present in such devices. Irradiation of this layer with charged particles, for example electrons, leads to the formation of quasi-stationary ions, and thus to the formation of intermediate conditions (F centers) in the band gap of the SiO.sub.2. These traps result in an increased dark current, and also degrade the charge transfer efficiency. Such charging effects also result in a modification of the shift potential at the gates.
In backside-thinned CCDs, the substrate on which the active layers are applied, in an epitaxial process, are substantially completely removed in a complicated, expensive process. In such backside-thinned CCDs, however, it is possible to allow the CCD to be exposed to radiation at the thinned side with short-range particle beams, for example electrons in the keV range, because the electrons are completely decelerated in the backside layer, and thus do not have a negative effect on the insulating layer. Such CCDs, however, are relatively expensive and are not consistent in quality.